The continued development of more powerful aviation turbine engines has demanded greater thermal stability of the fuel as a high temperature heat sink. This in turn requires better definition of the thermal stability of jet fuels, while still continuing to maintain high color quality. Thermal stability refers to the deposit-forming tendency of the fuel. Thus, it can be highly desirable that fuels for aviation turbine engines have sufficient thermal stability to prevent excessive deposits in the these powerful engines, as well as being relatively low in nitrogen content, while also exhibiting high color quality.
Although a number of refining techniques have improved thermal stability, most have drawbacks. For example, extraction methods with sulfuric acid, caustic, or SO2 have waste disposal problems. Uses of absorption/adsorption methods with agents such as silica gel or alumina have had marginal success. Clay adsorption has reduced capacity/applicability for jet fuels derived from heavier crude sources, and generally requires large quantities of material.
U.S. Pat. No. 4,906,354 discloses a process by which the thermal stability of jet fuel sweetened by an oxidation process can be improved by washing the sweetened fuel with caustic. More specifically, the method the thermal stability of the jet fuel sweetened by oxidation, as measured by the JFTOT test, comprises washing the sweetened jet fuel with aqueous caustic, washing the caustic-extracted jet fuel with water, and drying the water-washed jet fuel.
Statutory U.S. Invention Registration No. H1368 describes a method for improving the long-term color stability of jet fuel and jet fuel blends containing nitrogen compounds by intimately mixing the jet fuel with a quantity of concentrated sulfuric acid sufficient to remove at least 90% of the nitrogen compounds during contact time equal or less than 5 minutes, separating the jet fuel from the concentrated sulfuric acid, mixing the jet fuel with an aqueous caustic solution to remove residual acid from the jet fuel, separating the jet fuel from the aqueous caustic solution, mixing the jet fuel with water, and separating the jet fuel from the water.
U.S. Pat. No. 4,912,873 relates to the treatment of diesel or jet fuel with a non-ionic, macroreticular, cross-linked, acrylic aliphatic ester resin such as XAD-7 that reduces polar impurities and diesel color. The diesel or jet fuel samples are analyzed by the “floc test” which measured the amount of floe visually observed on contact with an aqueous iron solution containing 5 mM ferric sulfate in 5 nM sulfuric acid.
U.S. Pat. No. 2,267,458 relates to a process for refining hydrocarbon oils containing objectionable sulfur, color, and gum-forming compounds. The process comprises subjecting the oil to treatment with used sulfuric acid, which has been obtained from the alkylation of isoparaffins with olefins in the presence of strong sulfuric acid, whereby such objectionable compounds are substantially removed.
U.S. Pat. No. 3,487,012 relates to a process for the improvement of initial color and long-term color stability of aromatic concentrates. The process is considered to improve both initial color and long-term color stability of aromatic concentrates boiling between 400 and 750° F. without substantially reducing the aromaticity. The process comprises hydrotreating, acid treating followed by caustic washing, and vacuum distilling aromatic concentrates at 5 to 250 mmHg absolute pressure with corresponding temperatures in the range from 150 to 650° F.
U.S. Pat. No. 4,409,092 is directed to a combination process for upgrading hydrocarbon fractions obtained from raw shale oil, oil products of coal processing and select fractions of crude oils comprising sulfur, nitrogen, and metal contaminants to produce jet fuel product fractions such as JP4, JP5, JP8 and other turbine-type fuel materials. The combination process involves hydrotreating, acid extraction of basic nitrogen compounds, and hydrofining. A catalytic cracking process is also used to convert high-boiling portions of the hydrocarbon feed fractions to product boiling in the desired jet fuel boiling range, before acid extraction of basic nitrogen compounds. Thus, the combination process is indicated as maximizing the yield of desired jet fuel products under hydrogenating conditions, particularly conserving the consumption of hydrogen.
PCT Publication No. WO 2003/091361 discloses a process for improving the thermal oxidative stability of a distillate fuel such as jet fuel. The thermal oxidative stability is improved by adsorbing N—H containing heterocyclic compounds, such as indoles and pyrroles, with an adsorbent material. The adsorbent material includes compounds having a benzaldehyde functionality supported on a suitable support. The preferred compound is 4-aminobenzaldehyde, with the preferred support being clay.
U.S. Pat. No. 7,473,351 discloses a process for reducing the nitrogen content of a liquid hydrocarbon feed such as diesel or jet fuel. The feed, which comprises an alkylating agent such as an olefin and an organic nitrogen species, is contacted with an acidic catalyst at elevated temperature in a first reaction zone to generate a liquid hydrocarbon product comprised of a reduced amount of the alkylating agent and an organic nitrogen species of higher boiling point. The alkylating agent and higher boiling point nitrogen species are separated out by fractionation. The acidic catalyst can be a liquid or solid catalyst, with solid acidic catalysts being preferred. Solid acidic materials may comprise acidic polymeric resins, supported acids and acidic inorganic oxides. There is no indication, however, that the catalysts can be used to treat the fuel in such a manner as to also exhibit high thermal stability and color quality.
U.S. Patent Application Publication No. 2011/0131870 discloses a process for increasing color quality and thermal stability of fuel. Fuel that is provided as a feedstock is contacted or treated with an acidic, ion-exchange resin to increase the color quality and stability of the fuel. The process provides the benefit of substantially increasing the long term quality of both color and oxidation (JFTOT) stability.
Additional methods for upgrading fuels, including enhancing color quality and stability of fuels, are needed. Additional reduction in nitrogen content can also be desired. These qualities can be particularly desirable in locations where hydroprocessing volume can be limited. In particular, more simple processes using more readily available materials as catalysts to assist in such processing are highly desired.